Scanning coils are known which are capable of providing a signal that is a function of the depth of surface flaws when passed over said flaws (such as cracks, etc.) with lengths exceeding the dimensions of the scanning coil, provided the flaw is normal to the axis of the receiver winding in the scanning coil. If, however, such a scanning coil is passed over an extended crack with the axis of the receiver winding coinciding with the direction of the crack, either no signal at all is produced in the event the crack is passed over in a lateral direction, or a signal is produced at the beginning and end of the crack only when the direction of coil travel is parallel to the crack.
In spite of this limitation, scanning coils of the type described are widely used in cases where signals that are strict by a function of the depth are required of long cracks and when these cracks generally run in the same direction (i.e., in the case of rolling stock). The reason is that the design and adjustment of such scanning coils is very simple as compared to differential coils so that large areas can easily be inspected by arranging a number of coils in a line and closely adjacent to each other.
These scanning coils, however, cannot be employed for cracks of any angular orientation in cases where signals corresponding to the crack depth are desired. It was found in the case of the known types of scanning coils described above that the flaw signal is reduced to half its magnitude if the crack angle deviates even 17.degree. from the optimum position. If the deviation from the optimum crack position is 30.degree., the signal magnitude is only 20% of the signal obtainable for the optimum position.
Frequently the flaw signals are used for crack classification, i.e. for categorizing the flaws in accordance with their seriousness. The measures to be taken then depend upon the category. A prerequisite for this classification, however, is a clear relationship between crack depth and the resulting crack signal. Deviations from the optimum crack position as described above are no longer acceptable.